Devices that require electric power are commonly powered by a power cord connected to a wall receptacle or by a battery power supply. Power cords are often undesirable because they reduce the portability of such devices. Battery power supplies permit devices to be portable, but still require at least occasional replacement or recharging of the batteries. When recharging, the devices are often connected to a wall receptacle or placed into a power cradle, which is itself connected to a wall receptacle. Accordingly, even when a battery power supply is used, connection to a wall receptacle or other external power source is often still required in order to charge the batteries. Devices that do not require charging or connection to an external power source are desired.